Simplified selectColumnSizeHeuristic

threads.js has better support for modules - no need to give a
project-relative path to the worker file, which complicated the build.

Add rudimentary thread pooling w/ execution timeout.

src/sudoku/worker.js

@@ -5,12 +5,9 @@ const maths = {};
 
 expose({
   generate(regionWidth, regionHeight, clues) {
+    const key = `${regionWidth}:${regionHeight}`;
     const math =
-      maths[`${regionWidth}:${regionHeight}`] ||
+      maths[key] ?? (maths[key] = new SudokuMath(regionWidth, regionHeight));
-      (maths[`${regionWidth}:${regionHeight}`] = new SudokuMath(
-        regionWidth,
-        regionHeight
-      ));
     return math.generate(clues);
   },
 });